Two months do most of the work: June and September. What makes the question unusual here is that it is only half about weather — this island runs on a season, and much of what people arrive for is simply shut outside it.
The season is the real variable
Mykonos wakes in stages through spring and shuts down across October. Beach clubs, the better kitchens and a good deal of the boat traffic operate on that calendar, so a warm week in April and a warm week in June are not the same island in any meaningful sense. Turning up before the season has properly started buys you the lanes, the light and the archaeology, and almost none of the thing the island is known for.
| Stretch of the year | The island you get | The catch |
|---|---|---|
| April into May | Quiet lanes, green hills, low rates | Much still closed, sea too cold for most |
| June | Everything open, warm sea, crowds manageable | Rooms go early to people who know this |
| July and August | Peak Mykonos, at full volume | Heat, maximum crowd, wind at its strongest |
| September | Warm water, softer edges, season still running | Thins noticeably toward the end |
| October onward | Beautiful and nearly empty | Season over, most venues closed |
June or September
September holds more heat in the water and slightly less pressure everywhere else. Crowds have started to disperse, the main town is walkable again after dark, and the light improves week by week. June arrives fresher, with green still on the hills and a lower chance of the summer wind blowing hard enough to make the exposed beaches pointless.
Availability is the practical difference. June sells out earlier than people expect, because it has become the knowing choice; September has more give in it, particularly after the first fortnight. Neither month is quiet in absolute terms — they are only quiet against what happens between them.
The wind is a scheduling problem
The summer wind here is strong enough to close the exposed side of the island for a day or two at a time, and it blows hardest at the height of the season. That does not ruin a holiday. It means the beach you planned on may be unusable while another is glassy, so which coast you are based on matters, and that deserves its own look before you book anything.
The fortnight to avoid
The middle of August, without much qualification. Heat, maximum crowd, the largest Greek domestic holiday of the summer and the year's highest prices all arrive together. Anyone obliged to travel then should plan around the wind and reserve everything — boats, tables, transfers — a long way ahead.
If you only get one week
Take the second or third week of September. It is the most reliable combination this island offers: warm sea, a season still fully running, and a version of Mykonos where you can hear the person across the table.
Because the room and the month are effectively one decision here rather than two, comparing current stay and experience options before fixing dates is unusually worthwhile.